1. FIELD OF THE INVENTION
This invention relates to a logic circuit board provided with a clock observation circuit, and particularly relates to a logic circuit board enabling minimization of clock skew in an electronic device comprising a plurality of logic circuit boards.
2. DESCRIPTION OF THE PRIOR ART
Suppose a circuit device is provided with many logic circuit boards comprising a plurality of logic circuit IC chips and a clock supply IC chip to supply clock signal to such logic circuit IC chips mounted on a single printed wiring board. When such a logic circuit device is operated, it is required to observe clock skew at every logic circuit board and decide the clock cycle for the whole circuit device corresponding to the largest clock skew.
Conventionally, a logic circuit board of this kind is provided with an external terminal for observation from an IC chip at a typical location on a logic circuit board, and the output from such a terminal is used for clock observation. Alternatively, a clock supply IC chip is designed to have a wiring pattern of a certain length and the output from such pattern to the external observation terminal on the border of a printed wiring board is used for observation.
In such a conventional clock observation circuit as described above, since the clock is observed at an external observation terminal connected with an IC chip at a typical location or an external terminal connected with a clock supply IC chip, there is only a little likelihood that the point of clock observation (clock phase) for adjustment represents the true central value of the phase shifts at a plurality of IC chips receiving the clock on the logic circuit board. As a result, a circuit device consisting of many logic circuit boards is accompanied by a large amount of clock skew, which is difference in clock shifts among the logic circuit boards. Therefore, for a circuit device consisting of many logic circuit boards, it is necessary to prolong the whole clock cycle to cope with increase in clock skew among the logic circuit boards, which may lead to deterioration in device performance.